The mammalian immune system provides a means for the recognition and elimination of tumor cells, other pathogenic cells, and invading foreign pathogens. While the immune system normally provides a strong line of defense, there are many instances where cancer cells, other pathogenic cells, or infectious agents evade a host immune response and proliferate or persist with concomitant host pathogenicity. Chemotherapeutic agents and radiation therapies have been developed to eliminate, for example, replicating neoplasms. However, many of the currently available chemotherapeutic agents and radiation therapy regimens have adverse side effects because they work not only to destroy pathogenic cells, but they also affect normal host cells, such as cells of the hematopoietic system. The adverse side effects of these anticancer drugs highlight the need for the development of new therapies selective for pathogenic cell populations and with reduced host toxicity.
Researchers have developed therapeutic protocols for destroying pathogenic cells by targeting cytotoxic compounds to such cells. Many of these protocols utilize toxins conjugated to antibodies that bind to antigens unique to or overexpressed by the pathogenic cells in an attempt to minimize delivery of the toxin to normal cells. Using this approach, certain immunotoxins have been developed consisting of antibodies directed to specific antigens on pathogenic cells, the antibodies being linked to toxins such as ricin, Pseudomonas exotoxin, Diptheria toxin, and tumor necrosis factor. These immunotoxins target pathogenic cells, such as tumor cells, bearing the specific antigens recognized by the antibody (Olsnes, S., Immunol. Today, 10, pp. 291-295, 1989; Melby, E. L., Cancer Res., 53(8), pp. 1755-1760, 1993; Better, M. D., PCT Publication Number WO 91/07418, published May 30, 1991).
Another approach for targeting populations of pathogenic cells, such as cancer cells or foreign pathogens, in a host is to enhance the host immune response against the pathogenic cells to avoid the need for administration of compounds that may also exhibit independent host toxicity. One reported strategy for immunotherapy is to bind antibodies, for example, genetically engineered multimeric antibodies, to the surface of tumor cells to display the constant region of the antibodies on the cell surface and thereby induce tumor cell killing by various immune-system mediated processes (De Vita, V. T., Biologic Therapy of Cancer, 2d ed. Philadelphia, Lippincott, 1995; Soulillou, J. P., U.S. Pat. No. 5,672,486). However, these approaches have been complicated by the difficulties in defining tumor-specific antigens.
Tubulysins are a group of potent inhibitors of tubulin polymerization. Tubulysins are useful in treating diseases and disease states that include pathogenic cell populations, such as cancer. Two particular species of mycobacteria synthesize tubulysins in high titer during fermentation. One species, Archangium gephyra, produces as the main component factors tubulysins A, B, C, G, and I, each of which is characterized by a including the tubutyrosine (Tut, an analog of tyrosine) residue. In contrast, another species, Angiococcus disciformis, produces as the main component factors tubulysins D, E, F, and H, each of which is characterized by a including the tubuphenylalanine (Tup, an analog of phenylalanine) residue. Such bacterial fermentations are convenient sources of tubulysins.